In medical imaging the diagnosis is usually made by radiologists. When the images are available in computer readable form (digitized), the analysis of the images may be performed while they are displayed on a video monitor. Scientists in the medical diagnostic imaging field are continuously working to improve the quality of the images, acquired by the various modalities in the field. One of the well-known methods for improving the detectibility of leisions of a displayed organ is by "windowing".
Windowing describes the process of displaying the detected data by extending certain detected data ranges to fill the entire intensity range of the displayed image. For example, there are only a limited number of gray levels which may be distinguished, this being due to inherent limitations of the CRT and the human eye. Therefore, it is very important to utilize the available gray levels in the regions of data values which carry the most useful information. To illustrate, assume that a black and white system has the capability of displaying 256 gray levels. If the acquisition of data occuring during an examination of a subject results in only 86 gray levels clustered away from the lowest and the highest levels; then, only about one-third of the system's contrast display capability would be used.
The originators of "windowing" recognized that "spreading" the data over the 256 levels, that are available instead of displaying the data only over the acquired 86 levels would greatly enhance the contrast resolution. More particularly, the significant data may reside in a band of values, say 1000-1100. Rational utilization of the "gray level resource" is to assign the data value 1000 to the gray level 0, the data value 1100 to gray level 255 and intermediate data values to their proportionally corresponding gray level values in the range 1-254. In the example given, the contrast would be enhanced by a factor of about 3 to 1.
Windowing is presently accomplished by using a table-look-up or look-up tables. When displaying an image, the data is stored in a digital memory, and is read out repeatedly to refresh the video display. For that purpose special circuits are used to control the read out sequence, to synchronize the raster monitor timing, to convert the digital data into analog data, etc. These circuits are well known to those skilled in the art.
To perform the "windowing" function using table look up techniques, a translation table is stored in a memory within the electronic circuitry responsible for the display refresh. The image data, on a pixel-by-pixel basis is used as an address and applied to the memory. The corresponding gray level as stored in the memory is readout and used to control the intensity of the displayed pixel.
While the table look up based windowing technique may be an acceptable solution for relatively simple displays it does not provide a good solution for more advanced display systems. For example, in computerized tomographic (C.T.) imaging the following conditions adverse to prior art windowing techniques are or may be encountered:
The data may be represented by 12 bits PA1 The display system matrix may be a 1024.times.1280 unit PA1 A split screen may be used (subdividing the 1024.times.1280 matrix into say twenty (20) 256.times.256 images) PA1 Each of the 20 images ideally requires a different window setting PA1 The raster display is refreshed at a rate of (60 frames/sec.) in order to be flicker free
Under such conditions video signals in the range of 100 Megapixels/sec are needed to support the video rate. The memories currently used for table look up may operate at a cycle time of about 75 nanosecs. Hence, eight parallel channels are required to support the required rate. Also note that 20 (images).times.8 (channels).times.4K (length of table)=640 Kilobytes. With current technology, this would require 320 4K static memory components, each with a 4 bit capacity.
This high number of memory components plus the associated support electronics makes the video rate windowing based on table look up devices impractical, at least for the more advanced display systems as described above.
Accordingly, there is a need in the field of imaging for equipment and procedures that will accomplish the windowing function in a new and improved manner that does not require such a vast number of memory components.
An objective of the present invention is to provide efficient and economical equipment and methods for the performance of windowing as required for video rate windowing in raster refresh video systems, in general, and for high resolution sophisticated units in particular.